Gain control circuits



March 4-, 1941. ROBERTSON 2,233,487

GAIN CONTROL CIRCUITS Filed Nov. 18, 1939 INVENTOR D. 0. ROBERTSONAfro/PM? Patented Mar. 4, 1941- UNITED STATES PATENT OFFICE.

GAIN CONTROL CIRCUITS Application November 18, 1939, Serial No. 305,091

11 Claims.

This invention relates to signal transmission systems and particularlyto gain control circuits for controlling the volume on signaltransmission systems. I

I One object of the invention is to provide control circuits having arectifier and an amplifier with the rectifier controlled by receivedsignals and the amplifier controlled by the rectifier that shall have anegative feedback loop connected to the amplifier for eliminating highfrequency currents and for providing efficient terminating impedance forthe rectifier circuit.

Another object of the invention is to provide a signal transmission linewith control circuits having a diode-triode tube with the diodecontrolled by signals on the transmission line and the triode controlledby the diode that shall have a negative feedback loop connected to thetriode for eliminating high frequency currents and for providingefficient terminating impedance for the diode circuit.

A further object of the invention is to provide gain control circuitsfor a signal transmission line with a gain increaser circuit having adiodetriode tube with the diode controlled by signals on thetransmission line and the triode controlled by the diode that shall havea negative feedback loop connected to the triode for eliminating highfrequency currents while preventing extended hang-over in the operationof the triode and for providing efiicient terminating impedance for thediode circuit.

On many signal circuits, as for example on circuits having a long radiolink, trouble is often experienced at a receiving station by reason ofthe so-called fading produced during transmission. It is desirable incircuits of this type to correct for the fading and to hold the receivedsignals at substantially constant volume.

In a gain control circuit constructed in accordance with the inventionthe volume of the signals is held constant under control of a gainincreaser circuit and a gain decreaser circuit. The gain increasercircuit is operated to effect increase in the gain in case of lowstrength signals under control of syllabic frequency impulses. The gainincreaser circuit is forward acting and employs diode-triode tubeshaving a negative feedback 50 loop connected to the triodes to effectefficient operation of the diodes and to eliminate relatively highfrequency currents from the gain increaser circuit. The gain decreasercircuit is backward acting. The circuits are improvements on-the 55circuits disclosed in the H. L. Barney et a1. application Serial No.284,584, filed July 15, 1939, now Patent No. 2,213,034, issued August2'7, 1940.

A vario-repeater is located at a receiving station on the transmissionline for varying the gain. The vario-repeater may be in any suit- =5-able form and preferably comprises a .pair of push-pull tubes of anysuitable type and a varistor. The varistor preferably comprises a bridgeof copper-oxide elements shunted across the transmission path. Thecontrol grids of the 10 two tubes in push-pull relationships arecontrolled according to the charge on two control condensers. One of thecontrol condensers is relatively small compared to the other condenser.The varistor in the form of a copper-oxide bridge is controlledaccording to the space current flowing through the tubes in push-pullrelationship. When the tubes have large impedance and the space currentflow through them is small, the shunting action by the copper-oxidevbridge is large. When the tubes have small impedance andthe spacecurrent flow through themis large, the shunting action effected by thecopper-oxide bridge is small.

The backward-acting gain decreaser circuit is provided for impressing anegative charge on the control condensers to lower the gain on thetransmission line. The forward-acting gain increaser circuit impresses apositive charge on the control condensers to raise the gain on thetransmission line. The gain decreaser circuit and the gain increasercircuit are controlled according to the energy level of the signals onthe transmission line. I

The gain decreaser circuit which is connected to the transmission linebeyond the variorepeater comprises a high-pass filter which prevents theoperation of the gain decreaser circuit by low frequency transients dueto unbalanced plate current in the vario-repeater. The low frequencytransients might otherwise cause excessive reductions of gain. Thefilter is connected to 'a rectifier tube by means of a transformer. Therectifier tube impresses a negative change on the control condensers togovern the operation '45 of the push-pull tubes in the vario-repeater.The push-pull tubes when operated by the control condensers also controlthe copper-oxide bridge to lower the gain on the transmission line.

The gain increaser circuit is connected to the transmission line beforethe vario-repeaterand comprises a suitable filter which selects currentshaving a. range of frequencies usually in center portion of the voicefrequency range." In systems of transmission where the voice currents ofdifferent frequencies are not displaced, the selected frequencies arebetween 800 and 1600 cycles per second. However, in systems where thevoice frequencies have been displaced from their normal position, as iscustomary in systems of primary transmission, the pass band on thefilter is correspondingly displaced. The filter is connected to anamplifier and the amplifier is connected to two diode-triode tubes. Theinput signals when rectified by the diode portions of the diode-triodessupply a negative bias for controlling the grids of the triodes. Anegative feedtifier elements which are preferably of the copper-oxidetype are connected in series and in shunt to the secondary winding ofthe transformer connected to the'syllabic frequency filter. Thecopper-oxide elements serve not only to insure the operation of the gainincreaser relay on the rising portion of a syllabic impulse but alsoserve to improve the operating characteristics.

The gain increaser relay when operated impresses a positive charge onthe control condensers to raise the gain on the transmission line. Again rate relay which is operated by the output from the triodes ofthe'diode-triodes is provided for reducing'the rate of gain increaseeffected by the gain increaser circuit whenever the signals on thetransmission line before the vario-repeater are above a predeterminedlevel. The gain increase is controlled by syllabic impulses on' the gainincreaser circuit in order to insure against false operation of the gainincreaser circuit by noise or static disturbances. The gain increase iscontrolled only on a rising portion of a syllabic impulse so that in sofar as possible the gain increase action may be performed during theinitial part ofa speech syllable. Thus when the gain is sufficientlyhigh the remainder of the speech syllable may operate the gain decreasercircuit and no further gain increase may take place on the final part ofthe syllable.

Thesingle figure in the accompanying drawing is a diagrammatic view ofgain control circuits" constructed in accordance with the invention.

Referring to the drawing, a' gain control arrangement l at a receivingstation is shown having input conductors 2 and 3 and output conductors 4and 5. The input conductors 2 and 3 are connected to a source of signalsvarying in intensity and which may be the output of a radio receiver(not shown). 4 and 5 may be connected to a telephone line or to aloud-speaker (not shown). It is desired to apply signals of relativelyconstant intensity to the telephoneline or loud-speaker. Avario-repeater 6 comprising two pentode tubes 1 and 8 and a copper-oxidebridge 9 are provided in the path between the input conductors 2 and 3and the output conductors 4 and 5 for controlling the gain. The tube 1comprises a control grid Ill, an anode I 2 and a cathode H. The tube 8The output conductors comprises an anode l3, a control grid [4 and acathode I 5.

The'copper-oxide varistor bridge 3 is composed of copper-oxide elementsshunted across the transmission path. The input conductors 2 and 3 areconnected to the varistor 9 by means of transformers l8 and I1.Resistance elements l8 and I9 are connected in seriesandresistanceelement 28 is connected in shunt between the secondarywinding of the transformer I6 and the primary winding of the transformerl1. Resistance elements 2! are disposed on each side of the bridge 9 anda resistance element 22 is connected in shunt to the bridge. Atransformer 23 connects the pad formed bythe Varistor 9 and theresistance elements 24' and 22 to the input circuits of the tubes 1 and8 in push-pull relationship. The output circuits of the tubes 7 and 8are connected by a transformer 24 to a negative feedback amplifier 25.The output circuit of the amplifier 25 is connected by hybrid coil 28 tothe output conductors 4and 5 and to a gain decreaser circuit 21. s l

Two control condensers 28 and 29 are provided for controlling the gridsl8 and'M of'the tubes 7 and 8. The condenser 28 is relatively smallcomparedto the condenser 29 and serves to effect quick changes in gainof short duration on the transmission path. The larger condenser 29serves to control the tubes .1 and 8 in the case of prolonged changes involume on the transmission path. The condenser 29 in series withresistance elements 33, 3| and 32 is connected inshunt to the condenser28 so that small charges impressed on the condenser 28 may betransferred A battery 35- supplies potential for energizing thecopper-oxide bridge 9. The circuit for energizing the bridge 9 may betraced from grounded battery 35 through a resistance element 38,

bridge 9 and ground return tov the battery 35. y

The gain decreaser circuit 21 comprises a filter 37 connected to thehybrid coil 28. The filter 31 comprises two condensers 33 and 38, aninductance 48and resistance element 4!. The filter 31 is connected by atransformer 42 to a rectifier tube 43. The rectifier tube 43is connectedto the control condenser 28. The circuit from the tube 43 for chargingthe control condenser 28 may be traced from grounded battery 44 throughresistance element 45, secondary windings of the trans former 42,rectifier tube 43, resistance element 46, resistance element 32,condenser 28, resistance element 38, battery 35 and ground return to thebattery 44. 'I"hiscircuit serves to charge the condenser 28 in a mannerto impress negative potential on the grids l8 and M of the tubes 1 and8. I

In case the signals on the transmission path beyond the vario-repeatertgo above a predetermined level, the rectifier tube 43 supplies povtential for charging the control condenser 28 with negative potential.The negative potential on the control condenser 28 is impressed on thecontrol grids l0 and'l4 of the tubes 1 and 8 to increase the impedanceof the tubes and decrease the space current. 'Thecopper-oxide bridge 9is controlled in accordance with the space current flow through thetubes 1 and 8. The circuit for the spacecurrent through the tubes 1 and8may be traced from the grounded battery 33 through choke coil 41,primary windings of the transformer 24 in parallel, anodes l2 and I3,cathodes H and I5, resistance element 48, resistance element 36, battery35 and ground return to the battery 33. The potential drop across theresistance element 36 formed by the space current flow through the tubes1 and 8 cooperates with the potential drop across the resistance element35 in circuit with the battery 35 to control the copper-oxide bridge 9.The current flow through the bridge 9 from the battery 35 lowers theimpedance of the'bridge across the transmission path. The current flowfrom the battery 33 through the tubes 1 and 8 and resistance element 36produces a potential drop across the resistance element which reducesthe effect of the battery 35 on the bridge. Thus, the bridge 9 iscontrolled according to the space current flow through the tubes 1 and8. If the space current flow through the tubes 1 and 8 is reduced byimpressing negative potential on the grids I and I4, as in the caseunder consideration, then the resistance of the bridge is reduced toincrease the shunting action thereof.

A gain increaser circuit 49 is provided for impressing a positive chargeon the control condensers 28 and 29 to increase the gain on thetransmission path efiected by the vario-repeater 5. The gain increasercircuit 49 comprises a filter 50 connected to a transmission pathbetween the transformers l6 and I1 and ahead of the vario-repeater 6.The filter 59 selects a frequency band within the voice frequency rangewhich is centered around a frequency of the order of 1200 cycles. Thefilter 50 is connected by a transformer to the input circuit of anamplifier 52 of any suitable type. The output circuit of the amplifier52 is connected by a transformer 53 to the diode parts of twodiode-triode tubes 54 and 55.

The diode-triode 54 comprises a control grid 58, an anode 51, a cathode58 and two auxiliary anodes 59 and 60. The diode-triode 55 comprises ananode 8|, a control grid 62, a cathode 83 and two auxiliary anodes 64and 65. The circuit through the diodes of the diode-triodes 54 and 55may be traced from one terminal of the secondary winding of thetransformer 53 through auxiliary anodes 59 and 69 and cathode 58 inparallel to auxiliary anodes 64 and 65 and cathode 63 through impedanceelement 66, resistance element 6'! and resistance element 68 to theother terminal of the secondary winding for the transformer 53. Thepotential drop across the resistance element 61 serves to control thetriode of the tube 54 and the potential drop across the resistanceelements 81 and 68 controls the triode of the tube 55.

The output circuit for the triodes of the tubes 54 and 55 may be tracedfrom a grounded battery 59 through anodes BI and 51 in parallel,cathodes 63 and 58, impedance element 66, coil 18 of a gain rate relay1|, primary windings of a transformer [2, battery 35 and ground returnbattery 68. The impedance element 66 is in the input circuits for thetriodes of the tubes 54 and 55 and also in the output circuits for thetubes 54 and 55. The impedance element 66 has a high impedance atrelatively high frequencies and a negligible impedance at lowfrequencies such as syllabic frequencies of 6 to'8 cycles. Thisimpedance element 66 forms a part of a negative feedback loop for thetriodes of the tubes 54 and 55 and serves to eliminate high frequencyourrents and to provide efflcient terminating impedance for the diodesof the tubes 54 and 55.

A condenser 13 connected across the primary winding of the transformer12 forms a filter for selecting impulses of syllabic frequency. Thetransformer 12 is connected to a gain increaser relay 14 which impressesa positive charge on the control condensers 28 and 29 to lower thenegative potential on the grids I0 and 14 of the tubes 1 and 8. The gainincreaser relay 14 comprises a holding coil 15, an operating coil 16,and an armature 11, which operates between the contact members 18 andIS. The gain rate relay 1|, which is controlled by the output of thediodetriode tubes 54 and 55 comprises a biasing coil 89, the operatingcoil 16, and an armature 8| which operates between contact members 82and 83. The. biasing coils I5 and 80 for thegain increaser relay andgain rate relay are energized from the battery 84 through a resistanceelement A copper-oxide resistor 86 is connected in shunt to thesecondary winding of the transformer 12 and a copper-oxide resistor 81is connected in series between the transformer 12 and the coil 16 of thegain increaser relay 14 to insure op mating the relay [4 on the risingportion of the syllabic impulse and to improve the operatingcharacteristic. The gain increaser relay when operated moves thearmature 11 from engagement with the contact member 19 into engagementwith the contact member 18. Upon en ga'gement of the armature 11 withthe contact member 18 a circuit is completed for impressing a positivecharge on the control condensers 28 and 29. This circuit may be tracedfrom ground through resistance elements 88 and 89, contact member 82,armature 8|, resistance element 99, armature", contact member 18,resistance element 9|, resistance element 3|, resistance element 32,condenser 28, resistance element 36, battery 35 and groundreturn toresistance ele ment 88. The resistance elements 88 and 89 in theabove-traced circuit have potential impressed thereon from the groundedbattery 84. The battery 35 is connectedthrough a resistance element 92to contact member 19 for impressing negative potential on the armature11 when in released position. The impressing of a negative charge on thearmature 11 of the relay 14 prevents the leakage of any positive chargetothe contact member 18 from the resistance elements 88 and 89. Aresistance element 93 provides a leak for the charge on the controlcondensers 28 and 29. The voltage finally arrived at is controlled bythe potential drop across a resistance element 94. v

The operating coil for the gain rate relay 7| is connected to the outputcircuits of the triodes in the tubes 54 and 55 in order to beoperated'in accordance with the strength of the signals on thetransmission line. The gain rate relay H is normally held in operatedposition with the armature 8| in engagement with the contact member 82.When the strength of the signals on the transmission line is raisedabove a predetermined point, the gain rate relay is released and thearmature 8| engages the contact member 83. Negative potential isimpressed on the grids 56 and 62 of the tubes 54 and 55'from thecoupling resistances 6'1 and 68 so that the output current from thetubes'54 and 55 is reduced as the strength of the signals on. thetransmission line increases. When the gain rate relay H .is released andthe armature 8| engages contact member 83,.the potential drop acrossonly the resistance element 88 is impressed'on the condensers 28 and 29upon operation of the gain control relay 14. By this means the rate ofgain change is varied according to the strength of the signals on thetransmission line.

When signals are received from the transmission path the filter 58selects the desired range of voice frequency components which areamplified by the amplifier 52. The output from the amplifier 52 isrectified by the diode portions of the diode-triode tubes 54 and 55. Thediodes are coupled to the input circuits of the triodes by means of thecoupling resistance elements 6! and 68 to supply negative bias to thecontrol grids of the triodes. The negative feedback loop for thetriodes, which includes the impedance element 66, serves to eliminaterelatively high frequency currents in the input circuits of the triodes.The negative feedback loop also serves to provide efficient terminatingimpedance for the diodes. The syllabic impulses selected from the outputcircuit of the tubes 54 and 55 control the operation of the gainincreaser relay 14 for charging the condensers 28 and 29 with potentialto lower the negative potential on the grids l8 and M of the tubes 1 and8. Lowering the negative potential on the control grids of the tubes 1and 8 increases the space current flow through the tubes and lowers theimpedance of the tubes. An increase in the space current flow throughthe tubes 1 and 8 lowers the shunting action of the copper-oxide bridge'9 across the transmission path. Each time the gain increaser relay 14is operated it impresses an increment of positive charge on the controlcondensers 28 and 29. This progressively decreases the negative chargeon the control condensers and on the grids l and M of the tubes 1 and 8.If the signals on the transmission line before the vario-r-epeater areabove a predetermined level, the gain rate relay H is operated todecrease the rate at which the control condensers 28 and 29 are charged.

Modifications in the circuits and in the arrangement and location ofparts may be made within the spirit and scope of the invention and suchmodifications are intended to be covered by the appended claims.

What is claimed is:

1. In a signal system, an amplifier having a negative feedback loop,impedance means in said feedback loop which varies with the frequency ofthe currents carried thereby, a rectifier terminated by the impedanceacross said feedback loop, said terminating impedance increasing therectifier efi'iciency without producing hangover in the amplifieroperation and eliminating high frequencies from the amplifier circuitand means for applying Waves of a frequency range including relativelyhigh frequency waves to said rectifier, said rectifier output includingfrequencies for'which said feedback loop has high and low loop gains.

2. In a signal system, an amplifier having a negative feedback loop,impedance means in said feedback loop which varies with the frequency ofthe currents carried thereby, a rectifier terminated by the impedanceacross said feedback loop, said terminating impedance being low atrelatively high frequencies and high at relatively low frequencies toincrease the efiiciency of said rectifier without hangover in theamplifier operation to eliminate high frequencies in the amplifiercircuit, and means for applying waves of a frequency range includingrelatively high frequency waves to said rectifier, said rectifier outputincluding frequencies for which said feed.- back loop has low and highloop gains.

3. In a signal system, a diode-triode having the input circuit of thetriode coupled to the diode circuit, a negative feedback loop for saidtriode, impedance means in said loop which varies with the frequency ofthe currents carried thereby, said diode being terminated by theimpedance across said feedback loop to increase the efi'iciency of saiddiode without hangover operation and to eliminate high frequencies inthe triode circuit, and means for applying Waves of a frequency rangeincluding relatively high frequency waves to said diode, said diodeoutput including frequencies for which said feedback loop has high andlow loop gains. 7

4. In a signal system, a diode-triode having the input circuit of thetriode coupled to the diode circuit, a negative feedback loop for saidtriode,

impedance means in said loop which varies with the frequency of thecurrents carried thereby, said diode being terminated by the impedanceacross said feedback loop and said impedance quencies and high atrelatively low frequencies to increase the efliciency of the diodewithout hangover, and means for applying waves of a frequency rangeincluding relatively high frequency waves to said diode, said diodeoutput including frequencies for which said feedback loop has low andrelatively high loop gains.

5. In combination, a vario-repeater for controlling the gain on a signaltransmission line, and a gain increaser circuit connected to said linefor governing said vario-repeater, said gain increaser circuitcomprising a diode-triode tube, the diode of said tube rectifying thesignals received from the transmission line, means for controlling thetriode according to the operation of the diode, means comprising anegative feedback loop for said triode, the impedance across saidfeedback loop having low impedance at relatively high frequencies andhigh impedance at relatively low frequencies for terminating the diodecircuit to increase the diode efficiency Without hangover action by thetriode and for eliminating high frequency. currents.

6. In combination, a vario-repeater for controlling the gain on a signaltransmission line,

and a gain increaser circuit connected to said line for governing saidvario-repeater, said gain increaser circuit comprising a diode-triodetube, the diode of said tube rectifying the signals received from' saidtransmission line, means for controlling the triode according to theoperation of the diode, means comprising a negative feedback loop forsaid triode to eliminate high frequency currents and to increase thediode elliciency without hangover operation, and means controlled by theoutput current from the triode to govern the vario-repeater.

7. In combination, a vario-repeater for controlling the gain on a signaltransmission line, and a gain increaser circuit connected to said linefor governing said vario-repeater, said gain increaser circuitcomprising a diode-triode tube, the diode of said tube rectifying thesignals received from said transmission line, means for controlling thetriode according to the operation of the diode, an impedance element inthe output and input circuits of the triode and the diode circuit forproviding a negative feedback, said negative feedback providingterminating impedance for the diode to increase the efiiciency thereofwithout hangover action by the triode and for across said loop beinglowat relatively high freeliminating high frequency currents, and meanscontrolled by the output current from the triode for governing saidvario-repeater to raise the gain on the transmission line.

8. In combination, a vario-repeater for controlling the gain on a signaltransmission line, and a gain increaser circuit controlled by thesignals on the line for governing said vario-repeater, said gainincreaser circuit comprising a diode-triode tube, the diode of said tuberectifying the signals received from said transmission line, means foroperating the triode according to the operation of the diode, animpedance element in the output and input circuits of the triode and thediode circuit for providing a negative feedback, said negative feedbackproviding high terminating impedance for the diode to increase theefficiency thereof without hangover action by the triode and eliminatinghigh frequencies from the triode, and means controlled by the outputcurrent from the triode for governing said variorepeater to raise thegain on the transmission line and for controlling the rate at which thegain change is effected.

9. In combination, a vario-repeater for controlling the gain on a signaltransmission line, and a forward-acting gain increaser circuit forgoverning said vario-repeater, said gain increaser circuit comprisingtwo diode-triode tubes, the diodes of said tubes rectifying the signalsreceived from said transmisison line, coupling means between the diodecircuits and the input circuits of the triode to control the triodes ofsaid tubes, an impedance element in the output and input circuits ofsaid triodes and the diode circuits for providing a negative feedbackloop, said feedback loop providing high terminating impedance for thediodes to increase the emciency thereof without hangover action andeliminating high frequency currents from the triode circuits, and meanscontrolled by the output current from the triodes for governing saidvariorepeater to raise the gain on the transmission line.

10. In combination, a vario-repeater for controlling the gain on asignal transmission line, a backward-acting circuit for governing saidvario-repeater to lower the gain on the transmission line and aforward-acting gain increaser circuit controlled by signals on the linefor governing said vario-repeater, said gain increaser circuitcomprising a diode-triode tube, the diode of said tube rectifying thesignals received from said transmission line, means for controlling saidtriode according to the operation of said diode, an impedance element inthe output and input circuits of the triode and the diode circuit forproviding a negative feedback at high frequencies from the triode to thediode to eliminate high frequency currents, a syllabic frequency filterconnected to the triode output circuit, means comprising a gainincreaser relay controlled by the output from said filter for operatingsaid vario-repeater, and means comprising a gain rate relay connected tothe triode output circuit for changing the rate of gain by the gain raterelay according to the triode output.

11. In combination, a vario-repeater for controlling the gain on asignal transmission line and a gain increaser circuit connected to saidline for governing said vario-repeater, said gain increaser circuitcomprising a diode-triode, the diode of said tube rectifying the signalsfrom the transmission line, means for impressing negative potential onthe grid of the triode according to the operation of the diode, meanscomprising a negative feedback loop connected to said triode foreliminating relatively high frequency currents from the triode inputcircuit and for increasing the efficiency of the diode while limitingthe hang-over time of the potential impressed on the grid of the triodeto a relatively short period,

DONALD D. ROBERTSON,

